The long-term objective of this project is to establish an animal model with a high incidence of fetal infection of the chimeric HIV-1/SIV, RT-SHIV. This animal model will be used to determine if combination anti-retroviral chemotherapy is more effective than monotherapy in prevention of virus transmission from mother to fetus. The first aim of this project was to identify the minimum intra-amniotic dose of the RT-SHIV that will result in 100% infection of the offspring. Six pregnant macaques (gestational age 123.5 1 2.3 days) have been inoculated intra-amniotically with the RT-SHIV virus, two animals at each of three concentrations undiluted, 1:10, and 1:100. Maternal and infant blood was collected for clinical and virological monitoring. All infants were delivered by C-section to minimize infection of the infant by maternal fluids. Five infants were euthanized at one month of age; one infant died of unrelated causes at 11 days. Analysis of the RT-SHIV in blood by coc ulture and by QC RNA-PCR indicated that all dams were infected, as were all infants except one inoculated with a 1:100 dilution. Cord blood from all animals except the dam of the infected infant was virus positive by QC RNA-PCR. Amniotic fluid from all animals, when obtained without contamination, was virus negative. Lymph nodes, spleen and thymus were virus positive by coculture in all but the uninfected infant. The second aim of the project is to identify the most potent clinically approved protease inhibitor to be used in the drug combination therapy, based on its in vitro potency to inhibit RT-SHIV replication. Work on these studies is now being completed. FUNDING NIH grant RR00166.